Fluidized bed bottom ash coolers are widely used in fluidized bed combustion technology. The bottom ash removed from fluidized bed combustors contains a significant amount of heat. Removal of the heat in the bottom ash reduces the temperature of the ash, thereby facilitating handling and disposal of same. Recovery of the heat in the bottom ash is also desirable in order to enhance the overall thermal efficiency of the fluidized bed combustion plant. Fluidization of the ash in the ash cooler sharply enhances heat transfer between the ash and the cooling medium which allows for the size of the ash cooler to be reduced.
Typical existing prior art fluidized bed bottom ash coolers for a circulating fluidized bed (CFB) boiler are shown in FIGS. 1, 2, 3 and 4. FIGS. 1 and 2 illustrate a typical fluidized bed bottom ash cooler 10 which is provided within a refractory-lined box or enclosure and supported off of boiler structural steel. In certain circumstances, and as illustrated in FIGS. 3 and 4, the ash cooler 10 is provided within a fluid-cooled (typically water and/or steam-cooled) enclosure formed of membrane tube wall panels. In both types of fluidized bed ash cooler 10 designs, the fluidized bed ash cooler 10 is still a structure separate from the CFB furnace 20, and separately supported off of the boiler structural steel. As shown in FIGS. 1-4, ash for cooling is transferred from the CFB furnace 20 to the fluidized bed ash cooler 10 via an air-assisted conduit 30 connected between the CFB furnace 20 and a lower part of the ash cooler 10. The ash is fluidized within the ash cooler 10, typically with fluidization air supplied through the bottom of the enclosure surrounding the ash cooler 10, whether refractory-lined or water-cooled. Cooling of the ash within the ash cooler 10 takes place through heat exchange between the (relatively) cold air provided for fluidization and the hot ash. The heated air is then conveyed back to the CFB furnace 20 via a conduit 40 connected to an upper part of the ash cooler 10. Cooled ash is discharged via a drain (not shown) at the bottom of the ash cooler 10. The ash cooler 10 may include heat absorbing surface, typically water-cooled tube banks 50, placed within the fluidized ash bed established within the ash cooler 10. In such a case, a bulk of the heat from the hot bottom ash transferred from the CFB furnace 20 into the ash cooler 10 would be absorbed by the cooling water circulated through the water-cooled tube banks 50 with the air provided into the ash cooler 10 primarily playing the role of the fluidizing medium.
While the existing ash coolers provide necessary ash cooling and enhance boiler efficiency by returning the heat absorbed from the ash back to the boiler system, the existing ash coolers have several shortcomings including: a complicated support structure, the need for high-temperature expansion joints to accommodate differences in thermal expansion between the ash cooler and the furnace, and complexity of solids transfer from the furnace to the ash cooler.